CI: Fix generate-docs action

Effectively revert 743117f080.

This action began failing recently. Updating the Dockerfile per the
action's repo did not fix this. Unpinning Sphinx fixed it.

.github/actions/generate-docs/action.yaml

@@ -52,7 +52,6 @@ runs:
         build_only: true
         target_branch: master
         target_path: '../home/_build'
-        pre_build_commands: 'pip install -Iv sphinx==5.1.1'
 
     - uses: actions/upload-artifact@v4
       with: